Learning Electronics

Temperature Sensitive Switch For Solar Collector

This circuit can be used to turn the pump on and off when a solar collector is used to heat a swimming pool, for example. This way the water in the collector has a chance to warm up significantly before it is pumped to the swimming pool. A bonus is that the pump doesn’t need to be on continuously. The basis of operation is as follows. When the temperature of the water in the solar collector is at least 10 °C higher than that of the swimming pool, the pump starts up.

The warm water will then be pumped to the swimming pool and the temperature difference will drop rapidly. This is because fresh, cool water from the swimming pool enters the collector. Once the difference is less than 3 °C the pump is turned off again. R10/R1 and R9/R2 each make up a potential divider. The output voltage will be about half the supply voltage at a temperature around 25 °C. C7 and C8 suppress any possible interference.

The NTCs (R9 and R10) are usually connected via several meters of cable, which can easily pick up interference. Both potential dividers are followed by a buffer stage (IC1a/IC1b). IC1c and R3, R4, R5 and R6 make up a differential amplifier (with unit gain), which measures the temperature difference (i.e. voltage difference). When both temperatures are equal the output is 0 V. When the temperature of the solar collector rises, the differential amplifier outputs a positive voltage.

This signal is used to trigger a comparator, which is built round an LM393 (IC2a). R7 and P1 are used to set the reference voltage at which the comparator changes state. R8 and P2 provide an adjustable hysteresis. R11 has been added to the output of IC2a because the opamp has an open collector output. A power switch for the pump is created by R12, T1 and Re1. D1 protects T1 against voltage spikes from the relay coil when it is turned off.

A visual indication of the state of the controller is provided by IC4 (UAA170), a LED spot display driver with 16 LEDs. The reference voltage for the comparator is buffered by IC1d and fed to input VRMAX of the UAA170. R20/D21 and R23/D22 limit the input voltages of IC4 to 5.1 V, since the maximum permissible input voltage to the UAA170 is 6 V. When there is no temperature difference, LED D20 turns on.

As the temperature difference increases the next LED turns on. The full scale of the LED bar is equal to the reference voltage of the comparator. This means that when the last LED (D5) of the UAA170 turns on, the comparator switches state. This is also indicated by D2. The power supply has been kept fairly simple and is built around a LM7812 regulator. The circuit is protected against a reverse polarity at the input by D3.

You have to make sure that the input to the regulator is at least 15 V, otherwise it won’t function properly. There are a few points you should note regarding the mounting of the NTCs. NTC R9 should be placed near the output of the solar collector. You should choose a point that always contains water, even when some of the water flows back a little. NTC R10 should be mounted inside the filter compartment (where it exists), which continually pumps the swimming pool water.

This will give a good indication of the temperature of the water. The way the circuit has to be set up depends how it has been installed and is very much an experimental process. To start with, set hysteresis potentiometer (P2) halfway. Then set the reference voltage to about 1.5-2 V with P1. On a sunny day you can measure the voltage difference to get an idea as to which reference voltage needs to be adjusted. The hysteresis setting determines how long the pump stays on for, which is until the minimum temperature difference has been reached.